If your battery doesn't last as long as it used to, then you likely just need a battery replacement.
All the iPhone models are quite a challenge to get into making it very difficult for the average consumer to replace the battery. That’s why we don’t offer a battery replacement kit for the iPhone 3GS like we do with most iPod models.
The iPhone 3GS batteries that we sell are the real OEM battery, and will work just fine with iOS4 (and the earlier iPhone operating systems).
This 1200mAh battery is guaranteed for ten years, so if it ever fails to keep a full charge during this period of time, let us know and we will give you a new one (if you purchased it to install yourself) or re-install one for you (if you sent your device to us for the installation) at no additional charge! No other website offers this kind of coverage.
Look at the power and value this battery provides for your iPhone 3G:

We offer a variety of
same-day shipping options from us to you, perfect for any level of urgency.
If you suspect your device has water or liquid damage, we suggest setting up an order for our
repair evaluation as it’s possible that a battery alone will not solve the problems that you’re having.
Limited-Time Offer! Enter FREESHIP in the coupon code field at checkout for FREE USPS First Class Mail shipping or apply this offer towards any other shipping method that you prefer!! Applies to the leg of shipping from us to you.
Want to send your device to us for this repair? Great! Learn more about the simple repair process here.